U.S. Pat. No. 5,552,788 describes an antenna arrangement and signal processing which may be incorporated into an aircraft collision avoidance system. Collision avoidance systems are described in U.S. Pat. Nos. 5,077,673, 5,157,615 and 5,388,047, assigned to the assignee of this application. The disclosure of U.S. Pat. Nos. 5,552,788, 5,388,047, 5,157,615 and 5,077,673 are incorporated herein by reference. One of the advantages of the system described in the '788 patent is a simplified arrangement for using multiple direction finding antenna assemblies. In particular, prior to the '788 patent there were systems with multiple directional antenna assemblies for direction finding purposes which used phase encoded directional information. Extraordinary efforts were therefore necessary in those systems to maintain phase balance between the different channels in the equipment. In the absence of such efforts, the directional information could be lost. One of the improvements described in the '788 patent was a modification to the signal processing so that, to a large extent, the direction or bearing information was encoded in amplitude as opposed to phase.
This greatly-simplified the complexity, and as a result limited the cost of the equipment.
As described in the '788 patent, a pair of directional antennas are employed, each pair consisting of a pair of monopole antennas and a hybrid circuit. Preferably, the axes of the pair of directional antennas are orthogonal to each other. One most preferred mounting arrangement is to have one of the directional antennas on an uppermost surface of the aircraft or host, with the other directional antenna mounted on a lowermost surface of the aircraft or host.
Regardless of whether or not the transmitter being located is above or below the host in altitude, one of the pair of directional antennas is in the "shadow" of the body of the host aircraft. If the transmitter being located is above the host aircraft, then the body of the host aircraft will shadow the lowermost directional antenna, and vice versa.
As described in the '788 patent, signals are derived from the pair of directional antennas, the signals are processed and then applied to a table with information allowing the bearing of the transmitter to be located based on the directional antenna signals.
In particular, each directional antenna developed a pair of signals, from the hybrid, whose amplitudes indicated bearing of the transmitter relative to the axis of the directional antenna. A bearing memory stored bearing information as a function of two pairs of inputs. The two pairs of inputs, each pair of inputs from one of the directional antennas, was then applied to the bearing memory so as to develop bearing information.